The present invention relates to an apparatus and method for surface treating vitreous material, for example, glass and glazed surfaces such as china or porcelain.
Vitreous surfaces have, particularly immediately following the formation of the surfaces, high free energy. The nature of the surface makes it susceptible to abrasion damage from other similar surfaces and to undesirable reactions with the atmosphere and/or other materials it may contact. One of the more troublesome and technically demanding aspects of this situation is the treatment of newly formed glassware. Food and beverage bottles, for instance, are quite strong, at least theoretically, when formed, but, unless protected, undergo strength degradation as a result of contact with other bottles and attack by the atmosphere. Moisture in the atmosphere is particularly detrimental to the newly formed, unprotected glass surface. When it is considered that these bottles are processed on high-speed filling and packing lines which constantly jostle the bottles together with substantial force, it will be appreciated that damage can occur as a result of the mutual contact between the bottles. In fact, pristine, treated glass bottles are unsuited for use in modern packing lines unless greatly increased quantities of glass are included to compensate for the damage in loss and strength.
As is known in the art, the problem of abrasion and atmospheric damage to glassware has been, in large part, solved by various surface treatments on the glass substantially immediately upon formation or at least prior to packing of the glass. One basic and most important facet of such treatments is the surface treatment of the glass, prior to annealing, with various metallic compounds which pyrolytically decompose upon contact with the heated, freshly formed glass to form metallic oxide surface layers on the glass or other vitreous material. Exemplary of the more widely used and better performing of these metal oxide treatments is the formation of tin oxide on glass surfaces by exposure to stannic chloride vapor. The tin oxide surface treatment when present in films less than that which would cause objectionable light interference and iridescence, produces a surface condition which is lubricous, highly resistant to abrasion damage and protective against atmospheric attack.
While stannic chloride does not decompose to form a tin oxide layer on the glass surface until exposed to temperatures on the order of 1000.degree. F., it has been found that the stannic chloride vapor is subject to degradation through exposure to moisture. For that reason, it has been common practice to bubble dry air through the stannic chloride liquid to form the stannic chloride vapor in an air carrier. When free of water, air has been found to be a substantially inert carrier gas for stannic chloride vapor. Further, it has been possible to conduct the dry air containing stannic chloride vapor to the heated glass surface without encountering substantial problems. For this reason, treatment of glass with stannic chloride has been widely accepted and is in standard commercial use.
Unfortunately, after the stannic chloride vapor is vented into a coating enclosure and passed over the glass surface, a substantial residue of stannic chloride vapor remains which has not been pyrolytically decomposed. As a result of the practicalities of the coating situation, such as moist air admitted into the enclosure with the ware to be treated, the vapor in the enclosure comes into contact with moisture. When so exposed, stannic chloride forms a sticky, objectionable material which has an obnoxious vapor, coats surfaces which it contacts and, further, tends to corrode and attack such surfaces. This results in plugged vents, corroded hoods, and precludes any possible recycle of the stannic chloride vapor. For these reasons, it has to be the practice to vent the stannic chloride vapor to the atmosphere after exposing it to the glass.
While the background has been presented with regard to glassware, this has been done because the problems involved in treating glassware with metallic oxide compositions are most demanding. Also, stannic chloride has been used as illustrative of the treatment gases. This has been done because of the widespread use of such treatment gas. However, the problems discussed exist in greater or lesser extent with regard to other vitreous surfaces and other treatment gases employed to produce metallic oxide surface treatments. These are the treatment gases which will be more widely illustrated below.